The present invention relates to radio systems, and more particularly to a method and apparatus for applying frequency hopping (FH) spread spectrum, in radiocommunication systems operating in the presence of interference.
In recent decades, progress in radio and VLSI technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radios, can now be produced having acceptable cost, size and power consumption.
Although radio communications technology today is focused mainly on voice communications, for example, handheld radios, cellular phones and the like, likely expansion in the near future promises to provide greater information flow to and from other types of nomadic devices such as mobile fax and mobile Internet browsers, and fixed devices such as computers and base stations. More specifically, it is likely that further advances in radio communications technology will provide very inexpensive radio equipment, which can be easily integrated into many kinds of devices. Advances in radio communications promises to reduce the need for cable-based services that are currently in favor. For example, master devices may communicate with respective peripherals over wireless links, thus eliminating the need for cables.
To accommodate the rapid expansion of radio communications services extensive bandwidth is required. An unlicensed band with sufficient capacity to allow for high data rate transmissions would be desirable. A suitable band for such an endeavor may be found in the ISM (Industrial, Scientific and Medical) band at 2.45 GHz, which is globally available. The band provides 83.5 MHZ of radio spectrum roughly centered at 2.45 GHz.
To allow different radio networks to share the same radio medium without coordination, signal spreading is usually applied. In fact, the United States Federal Communications Commission (FCC) presently requires commercial radio equipment operating in the 2.45 GHz band to apply some form of spreading when the transmit power exceeds about 0 dBm. Spreading can either be at the symbol level by applying direct-sequence (DS) spread spectrum or at the channel level by applying frequency hopping (FH) spread spectrum. The latter is attractive for the radio applications mentioned above since it more readily allows the use of cost-effective radios.
Spread spectrum using DS spreading involves modulating a carrier waveform with a data signal x(t). The data modulated carrier is then modulated further with a wideband spreading signal g(t) and transmitted. To decode the transmission a synchronized replica of the spreading signal g(t-Td), where Td is a receiver estimate of propagation delay, is remodulated with the received signal and, if properly synchronized (e.g., if the estimate Td is correct), a despread signal can be demodulated and recovered. In FH systems using M-ary Frequency Shift Keying (MFSK), k=log2M information bits are used to determine which one of M frequencies are to be transmitted for each “hop”. MFSK is a commonly used modulation method to achieve spreading. For more detailed information regarding direct sequence and frequency hopping spread spectrum, see Digital Communications, Fundamentals and Applications, Bernard Sklar, 1988, Prentice Hall, pp 550, et. seq.
Both spreading methods have their merits and drawbacks. Direct sequence (DS) spread spectrum, for example, has been able to deliver high, per channel raw data rates, but may be limited in the number of overlapping cells that can be, supported before a DS system begins to interfere with itself. Frequency hopping spread spectrum, conversely, does not suffer to the same degree from the self-blocking tendency of DS. Frequency hopping systems may support substantially more overlapping cells before system performance begins to degrade. However, the raw data rates delivered by FH systems have, historically, been substantially below that of DS. On balance however, given the present state-of-the-art, FH spreading results in less complex and cheaper radio transceivers than DS spreading.
The 2.45 Ghz ISM band is presently available for any system fulfilling the part 15 rules as required by the FCC. In Europe and Asia, similar rules are set by communications authorities. Systems operating in the ISM band therefore must deal effectively with a variety of sources of unknown and unpredictable interference due to the wide array of different systems which may transmit freely in the ISM band. Ironically, the most significant interference sources at present in the 2.45 GHz ISM band are not communication devices but are microwave ovens.
Microwave ovens presently operate in a band ranging from about 2.44 GHZ to 2.48 GHZ and must abide by separate FCC rules, namely FCC part 18, which rules are less stringent than those found within FCC part 15. FCC part 18, allows for higher signal powers and broader signal spectra thus increasing the likelihood of interference between, for example, microwave ovens and radio communication devices. Moreover, the interference from microwave ovens is relatively unpredictable compared to other interference sources and may depend on the make of the oven, the type of food being heated, and the like. Nevertheless, because of the nature of interferers like microwave ovens which may be present across the frequency spectrum special problems may arise which call for special solutions not readily found in prior art systems.
One such system is disclosed in U.S. Pat. No. 5,436,906 issued to Kasuya et al (hereinafter “Kasuya”). Kasuya discloses a radio telecommunication system which uses a non designated time slot when signal quality decreases below a quality criterion for a signal in a first time slot.
U.S. Pat. No. 5,436,906 to KASUYA et al. describes a TDMA communications protocol between a base-station and a mobile terminal. If one of the receivers (either in the base-station or in the mobile terminal) detects a higher bit error rate (BER) than a predetermined threshold a process will be initiated to use more than one time-slot. The system first checks if there are unused time-slots. If this is the case one or more additional time-slots will be allocated. Information is then sent on two or more time-slots. The receiver can then compare the BER of the different time-slots and use the one with the lowest BER. The extra time-slots can later be released if signal quality improves.
U.S. Pat. No. 5,663,957 to DENT describes a TDMA system which utilizes two time-slots when needed in order to maintain good received signal quality. See especially claims 1, 5 and 6. document 2 also mentions FH and TDD. See especially column 23, row 37 to column 24, row 23 and column, 3, rows 50 to 56.
U.S. Pat. No. 5,390,166 to ROHANI et al. describes a communication system operating according to frequency hopping code division multiple access (FH-CDMA). The receiver receives several time slots and either combines them or selects the best slot to produce an output signal.
U.S. Pat. No. 4,616,364 to LEE describes a digital hopped frequency, time diversity system that transmits a piece of information on different frequencies. The receiver selects the best signal.
While the above mentioned systems address certain issue and problems primarily in TDMA type systems, none address the impact of a time and/or frequency varying interferer on an FH channel. It would therefore be appreciated in the art for a method and apparatus for reducing the impact of an interferer on a FH communication channel.